The present invention relates to a piezoelectric element. More particularly, the present invention relates to a piezoelectric portion which has very high piezoelectric properties, and which has a high rate of increase of displacement as an actuator in the case of applying a large electric field and a high decomposability as a sensor in the case that a great force is applied.
In recent years, piezoelectric elements have been used in ink jet printer heads, speakers, microphones, etc.
As the piezoelectric elements, there are generally known those comprising a ceramic substrate, a piezoelectric portion made of a piezoelectric ceramic composition, formed on the substrate, and electrodes electrically connected to the piezoelectric portion. As to the piezoelectric ceramic composition constituting the piezoelectric portion, various improved compositions have been disclosed.
For example, a Pb(Mg1/3Nb2/3)O3—PbTiO3—PbZrO3 ternary system solid solution composition and a piezoelectric ceramic composition wherein part of Pb in the above composition is replaced by Sr, La, etc., were disclosed [Japanese Patent Publication S44-17103, Japanese Patent Publication S45-8145]. With regard to a piezoelectric portion itself which are the most important factor determining the piezoelectric element properties of the piezoelectric element, a piezoelectric element having excellent piezoelectric properties (e.g. piezoelectric d constant is expected.
When a piezoelectric element is produced actually by superposing a piezoelectric material made of the above piezoelectric ceramic composition on a ceramic substrate and then subjecting the piezoelectric material to heat treatment, the obtained piezoelectric portion has a low density, and therefore, there have been pointed out such a problem that the piezoelectric element has a low flexural displacement or, when a voltage is applied thereto, it causes dielectric breakdown at the low density area. In particular, this problem is remarkable in the case of a piezoelectric element having a structure that a negative electrode and a positive electrode are disposed between piezoelectric portions alternatively, and its improvement has been strongly demanded.
In addition, sufficient piezoelectric properties cannot always be obtained with a piezoelectric portion made of the aforementioned piezoelectric ceramic composition, and there have been such a problem that when a voltage is raised to increase a flexural displacement, an increase of the flexural displacement obtained is very small relative to an increase of voltage on a side of a high electric field of 4 kV/mm or more.
Hence, there has been proposed a piezoelectric element produced by previously heat-treating a piezoelectric material made of the above piezoelectric ceramic composition to produce a piezoelectric portion, and then attaching this piezoelectric portion onto a ceramic substrate (Japanese Patent Application H11-29357).
In this piezoelectric element, attention was paid to a fact that use of a ceramic substrate impairs the densification of a piezoelectric portion. Thus, the piezoelectric element was accordingly improved in piezoelectric properties by means of obtaining a dense piezoelectric portion where a piezoelectric material made of a piezoelectric ceramic composition was previously heat-treated.
In this piezoelectric element, however, it is necessary to use an inorganic or organic adhesive at the time of attaching the piezoelectric onto the ceramic substrate. Therefore, there have been such problems that the adhesive impairs the vibration transmittability between the ceramic substrate and the piezoelectric or the adhesive components infiltrate into the piezoelectric or the ceramic substrate, deteriorating their properties.
In addition, in this piezoelectric element, no consideration is given to a piezoelectric ceramic composition itself constituting a piezoelectric portion. Therefore, there has been a problem that sufficient piezoelectric properties cannot always be obtained like the aforementioned piezoelectric element, and further, an increase of the flexural displacement obtained is very small relative to an increase of voltage in a region of a high electric field.